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Science: Baby star found without a cradle

13 November 1993

By
KEN CROSWELL in
BEREKELEY

A maverick star is baffling astronomers. HD 98800 contains a large amount
of the element lithium – an indication that the star is very young. But
whereas most young stars are found among hundreds or thousands of others
in vast clouds of interstellar gas and dust, HD 98800 is alone.

The star, which is orange, first attracted attention earlier this year
when astronomers reported that 10 per cent of its light is emitted at far-infrared
wavelengths. This radiation comes from a disc of dust which forms a ring
around the star, and is heated by it. The existence of such a dusty disc
could indicate that the star is the centre of an embryonic planetary system
(New Scientist, Science, 20 March).

Now Francis Fekel of NASA’s Marshall Space Flight Center in Huntsville,
Alabama, and Bernard Bopp of the University of Toledo in Ohio have studied
HD 98800 more closely. Their observations reveal that the star is less than
10 million years old, which is only 1/500th the age of the Sun. The star’s
youth supports the idea of a dusty disc, because very young stars often
have such discs around them.

Fekel and Bopp base their conclusion on observations of the star’s lithium
content. Lithium is abundant in very young stars, but it is burnt up by
nuclear reactions as a star gets older. In April, Fekel and Bopp used a
telescope at Kitt Peak in Arizona to measure HD 98800’s spectrum, from which
they estimate that the star has 10 times as much lithium on its surface
as the Sun.

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In 1992, a team of Brazilian astronomers also reported that the star
contains lithium and classified HD 98800 as a T Tauri star (Astronomical
Journal, vol 103, p 549). These are newborn objects that are larger and
brighter than the Sun. As they age, T Tauri stars shrink and fade, becoming
main-sequence stars like the Sun. Main-sequence stars fuse hydrogen into
helium at their centres; T Tauri stars convert gravitational energy into
heat.

Ben Zuckerman and Eric Becklin of the University of California at Los
Angeles, who earlier this year claimed that HD 98800 had a disc of dust,
were unaware of the work by the Brazilian astronomers and classified the
object as a main-sequence star.

Fekel and Bopp, however, believe that neither classification is correct.
Instead, they say, HD 98800 is in transition from the T Tauri stage to the
main sequence. The star’s spectrum is not like that of a T Tauri star, but
the star is still so young that its hydrogen has yet to ignite.

The change in the star’s status affects estimates of how far away
it is. No one has measured the distance directly, so astronomers infer
it by comparing the star’s apparent brightness with what they believe is
its intrinsic brightness. The greater the intrinsic brightness, the farther
the star. If HD 98800 were a main-sequence star, it would be about 65 light
years away. But because it is so young, Fekel and Bopp believe that the
star is intrinsically brighter, and is about 110 light years from Earth.

This puts HD 98800 hundreds of light years from any known region of
star formation, the nearest of which is 500 light years from Earth. One
possible explanation for HD 98800’s odd location is that the star was born
in a normal region of star formation and was ejected. However, Fekel and
Bopp say that HD 98800 is not fleeing any known region of star formation.

An alternative explanation, they say, is that the star was born where
it is, in a tiny region of star formation that has since dispersed. The
star is in the constellation of Crater, and the Brazilian astronomers who
had studied HD 98800 noted that a T Tauri star lies 10 degrees from HD 98800.
If the two stars are equidistant from Earth, they would be about 20 light
years apart.

Three other young stars lie within 7 degrees of TW Hydrae. These stars,
plus HD 98800, may be the remnant of a miniature star formation region that
gave birth to a few stars a few million years ago.

Fekel and Bopp will report their work in the 20 December issue of Astrophysical
Journal Letters.